Heat transfer duct

ABSTRACT

A heat transfer duct, such as a cooling duct, for use in cooling the slow wave circuit of a microwave tube includes a heat transfer structure disposed within the duct for improving the thermal transfer between the fluid and the inside walls of the duct. The heat exchanging structure comprises a strand of thermally conductive material wound into a first coil of relatively small diameter. That coil in turn is wound into a larger coil which in turn is wound into still a larger coil to form a coiled-coiled-coil geometry within the duct.

United States Patent [1 1 Wilczek HEAT TRANSFER DUCT [75] Inventor: Andrew S. Wilczek, Beverly, Mass. [73] Assignee: Varian Associates, Palo Alto, Calif.

[22] Filed: Jan. 10, 1973 [21] Appl. No.: 322,315

[52] U.S. Cl 315/3.5, 138/38, 165/179, 313/30 [51] Int. Cl. I-IOlj 25/34 [58] Field of Search 315/35; 313/30; 138/38;

[56] References Cited UNITED STATES PATENTS 3,595,299 7/1971 Weishaupt et al. 138/38 3,666,983 5/1972 Smith 313/30 3,329,855 7/1967 Landsbergen.... 315/35 3,617,798 11/1971 Marchese 315/35 2,500,501 3/1950 Trumpler 138/38 X FOREIGN PATENTS OR APPLICATIONS 1,238,462 7/1971 Great Britain 317/234 [4 Mar. 26, 1974 Primary Examiner-James W. Lawrence Assistant ExaminerSaxfield Chatmon, Jr.

Attorney, Agent, or FirmStanley Z. Cole; D. R. Pressman; Robert K. Stoddard [5 7] ABSTRACT 7 Claims, 5 Drawing Figures III/ [III

II/I/I/I/I HEAT TRANSFER DUCT GOVERNMENT CONTRACT The invention herein described was made in the course of a contract with the department of the US Army.

BACKGROUND OF THE INVENTION The present invention relates in general to cooling ducts particularly useful in microwave tubes and more particularly to an improved heat exchanging structure located inside the duct to provide a controlled porous metal insert for such cooling ducts.

Small tubular elements of slow wave circuits which are bombarded by the electron beam within a microwave tube, dissipate energy in the form of heat that can be carried away by forcing liquid coolant flow through the tubular elements (ducts). High heat dissipation densities result in local boiling at the inside surface of the wall of the duct which can cause a vapor barrier to heat flow with resultant burn through of the tubular elements from excessive heat rise.

Heretofore, this has been avoided by using high pressure, high velocity, turbulent coolant flow which carries away bubbles, thereby preventing the vapor barrier from being formed between the bulk coolant and the inside wall of the coolant duct. However, this technique limits the temperature rise of the coolant to that portion near the walls of the duct and consequently, the bulk temperature rise of the coolant is small. This in turn, can lead to the necessity for a large volume of coolant flow at high pressure resulting in large, high power pumping systems. It is more desirable to utilize a system which permits a greater bulk rise in temperature for the coolant requirements and to improve the efficiency of the heat exchange system.

A known technique for providing an improved heat exchange system is to use a porous metal insert in the tubular duct which has a twofold purpose. First, the impedance to the flow will cause great turbulance even at low flow rates. Secondly, the porous metal serves to conduct heat into the body of the coolant that has been transferred to the coolant near the wall. As a result, more heat is carried away by the coolant at a lower flow rate.

Slow wave circuits for crossed-field microwave tubes are often constructed using tubular elements when such tubes are intended for high average power operation. However, the tubular elements usually have small outside and inside diameters. Consequently, it is difficult to insert a properly controlled porous metal insert into the tubular element.

One technique that has been described in the literature utilizes a small diameter wire which has been cut into small lengths which are less than the inside diameter of the tubular element or duct. These are then dropped into the duct in a random fashion to form porous metal insert. This arrangement has been demonstrated to be able to carry away much more dissipated heat energy at a given coolant flow rate than can be done by the same arrangement without the porous metal insert. There are, however, practical difficulties with this arrangement. It is not possible to reproduce with accuracy the geometry of the porous metal insert. Hence, the thermal dissipation and heat transfer properties can vary from the element to element when constructed in this fashion, and also the heat transfer characteristics can vary over the length of any particular tubular element. Furthermore, the small pieces of metal wire can migrate as a result of coolant flow causing a compacting of the metal wire particles in one region and removal of the wire particles in another region. This leads to a marked variation in porosity as a function of time. Because of these practical limitations, this technique has not been utilized in actual microwave tubes. Therefore, it is desirable to provide an improved heat exchanging structure for use within small coolant ducts to obtain a more reliable and controlled porous metal heat exchanging structure for such ducts.

SUMMARY OF THE PRESENT INVENTION The principal object of the present invention is the provision of an improved heat transfer duct and electron tubes using same.

In one feature of the present invention, a heat transfer structure within a heat transfer duct comprises a strand of thermally conductive material, such as metal, wound into at least a coiled-coil configuration with the outer periphery of the heat exchanging structure abutting the inside wall of the duct to provide a thermally conductive joint between the outer periphery of the coiled-coil heat exchanging structure and the wall of the duct for improved heat transfer therethrough.

In another feature of the present invention, the heat transfer structure provided within the duct comprises a coiled-coiled-coil strand of thermally conductive material.

In another feature of the present invention, a microwave tube having an evacuated envelope containing an anode and cathode for forming an electron stream includes a wave energy supportive structure disposed in electromagnetic wave energy exchanging relation with the beam to produce an output microwave signal. A cooling duct is disposed in heat exchanging relation with the wave supportive structure, said cooling duct including a heat exchanging structure therein comprising a strand of thermally conductive material wound into at least a coiled-coil configuration to provide improved fluid cooling for the wave supportive structure in use.

Other features and advantages of the present invention will become apparent upon a perusal of the following specification taken in connection with the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a transverse sectional view of a crossed field microwave tube incorporating features of the present invention,

FIG. 2 is a longitudinal sectional view of the structure of FIG. 1 taken along line 2-2 in the direction of the arrows,

FIG. 3 shows the first step in the manufacture of a coiled-coil heat exchanging structure,

FIG. 4 shows the second step in the formation of a coiled-coil heat exchanging structure, and

FIG. 5 shows an enlarged view of a portion of the structure of FIG. 2 delineated by line 5-5 and depicting the improved heat exchanging structure of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIGS. 1 and 2, there is shown a microwave crossed field electron tube 11 incorporating features of the present invention. Tube 11 includes a hollow cylindrical evacuated envelope 12, as of copper. A cylindrical thermionic cathode emitter 13 is coaxially disposed of the envelope 12 in radially inward spaced relation therefrom. The cathode 13 is supported from the envelope 12 via the intermediary of an annular insulator 14 supporting a stem portion 15 of the cathode electrode. An array of one-half wavelength hollow cylindrical tubes 16 are spaced around the outside periphery of the cathode and form a coupled bar slow wave circuit operating at anode potential. A solid copper block 17 interrupts the circular array of bars 16 to form a circuit sever to define an input and an output end to the slow wave circuit.

Input radio frequency energy to be amplified is fed to the input end of the slow wave circuit 16 via input coaxial line 18 and amplified microwave energy is extracted from the output end of the slow wave circuit via output coaxial line 19. An axially directed magnetic field is provided in the space between the anode 16 and cathode 13 to cause electrons emitted from the cathode 13 to circulate around the cathode in electromagnetic wave energy exchanging relation with the slow wave circuit 16. A magnet for producing the axially directed magnetic field is not shown. Crossed field microwave amplifiers of the aforedescribed type are disclosed and claimed in US. Pat. No. 3,185,890 issued 25 May 1965 and assigned to the same assignee as the present invention.

In the energy exchanging process between the electron stream and the energy traveling on the slow wave circuit 16, the electron stream is collected on the slow wave circuit 16 which operates at anode potential, and thus a substantial amount of thermal energy is dissipated on the slow wave circuit 16. Heat is removed from slow wave circuit 16 by passing a fluid coolant, such as water, axially through the centers of the hollow bars 16. More particularly, input coolant fiow is supplied to an input annular fluid coolant manifold 21 via input coolant line 22. The coolant is directed from the input manifold 21 through the hollow interior or ducts ofthe circuit bars 16 to the disc shaped coolant collecting manifold 23 and thence extracted from the collection manifold 23 via exhaust coolant tubulation 24.

A porous metallic heat exchanging structure 25 is disposed in the cooling ducts within the circuit bars 16 for enhancing the transfer of heat from the bars 16 to the flow of coolant passing through the bars 16. In a preferred embodiment of the present invention, the heat exchanging structure 25 is shown in greater detail in FIG. 5 and comprises a coiled-coiled-coil heat exchanging structure threaded through the tubular bars 16.

As used herein coiled-coiled-coil" configuration means that a thin strand thermally conductive material such as metal wire 0.001 inches in diameter is wound into a relatively tight helix or coil of a first diameter as of 0.002 inches. This first helical strand or coil is then wound into a second coil having a larger diameter as of 0.006 inches in diameter. The coiled-coil is then wound again into a coil or helix of still larger diameter. The third coil or helix having a diameter as of 0.016 inches.

This resultant configuration is referred to herein as the coiled-coiled-coil configuration.

The coiled-coiled-coil heat exchanging structure 25 is then inserted within the ducts to be cooled. The coiled-coiled-coil heat exchanging structure 25 may be held within the duct merely by the outwardly tensioned forces on the spring-like coil structure or, in a preferred embodiment, the heat exchanging structure 25 is coated with a relatively low melting point material, as of copper, inserted within the duct, and then heated to brazing temperature for brazing the heat exchanging structure 25 within the inside of the duct to form a good thermally conductive bonded joint between the heat exchanging structure 25 and the inside wall of the duct.

In the case ofa microwave tube having relatively high thermal dissipation on the circuit bars 16, the circuit bars 16 are often formed of tungsten or molybdenum and the heat exchanging structure 25 is formed of a tungsten wire, as of 0.001 inches in diameter.

Coiled-coiled-coil tungsten wire is commercially available from incandescent lamp filament manufacturers. However, such a configuration can be readily formed in the manner as shown in FIGS. 3-5 and as follows: A strand 31 of thermally conductive material such as copper, tungsten, molybdenum or the like, as of 0.001 inches in diameter, is wound on a cylindrical mandrel 32 as of molybdenum wire of 0.002 inch diameter. The pitch for the coil 31 is selected based upon the required ultimate porosity. In other words, the less the pitch the less the porosity.

After winding the strand 31 on the mandrel 32 as shown in FIG. 3 the resultant structure is fired at elevated temperature to anneal and to set the coil configuration. This configuration is then wound on a second mandrel 33, as of molybdenum, of larger diameter such as 0.006 inches in diameter. Again the structure is annealed and set at elevated temperature.

This length of coiled-coil configuration is then wound on a still larger diameter mandrel 34 as shown in dotted lines in FIG. 5. In a typical example, mandrel 34 is 0.016 inches in diameter. This final coiled-coiled-coil structure is then annealed and set at elevated temperature. The molybdenum mandrels 32, 33 and 34 are then selectively etched away leaving the coiled-coiledcoil heat exchanging structure 25. In a typical example the final heat exchanging structure 25 has an outside diameter of for example 0.050 inches.

The coiled-coiled-coil heat exchanging structure 25 can be stretched slightly to become smaller in diameter for ease of insertion into a duct such as the interior of the slow wave circuit bars 16. In the case where the heat exchanging structure 25 is made of a tungsten strand, the heat exchanging structures are quite stable and attempt to return to their original shape upon release. Good thermal contact between the heat exchanging structure 25 and the inside wall of the duct 16 is obtained by making the inside diameter of the tubular element or duct equal to or slightly smaller than the outside diameter of the coiled-coiled-coil heat exchanging structure 25. In this case the heat exchanging structure is held in position by friction forces to obtain a mechanical thermally conductive joint between the duct and the heat exchanging structure. In a preferred embodiment, the heat exchanging structure 25 is plated with gold, inserted within the duct 16 and heated to elevated temperatures for melting the gold and forming a brazed joint between the heat exchanging structure 25 and the inside wall of the duct 16.

Such heat exchanging structures 25 can be made from other materials than tungsten and molybdenum. For example, the heat exchanging strand 31 may be made of molybdenum wire and wound on iron mandrels which can be selectively etched away after construction to leave the molybdenum coiled-coiled-coil heat exchanging structure 25.

The advantage of the coiled-coiled-coil heat exchanging structure 25 is that it is relatively easy to fabricate and produces uniform porosity and heat exchanging properties throughout the length of the duct. While the preferred geometry of a heat exchanging structure is a coiled-coiled-coil, other geometries may be envisioned. One in particular, is merely a coiled-coil such latter configuration, while not possessing all of the desirable properties of the coiled-coiled-coil configuration provides uniform porosity and improved heat exchanging capabilities and ease of manufacture.

What is claimed is:

1. In a heat transfer duct:

a duct for passage of a fluid therethrough in heat exchanging relation with said duct for transfer of heat therebetween; thermally conductive heat exchanging structure disposed in said duct in heat exchanging relation with the fluid to be passed through said duct and with the walls of said duct for increasing the heat transfer between the walls of said duct and the fluid; said heat exchanging structure comprising, a strand ofthermally conductive material wound into at least a coiled-ciled-coil configuration, the space between adjacent turns of said coil being substantially unfilled and said coiled-coil strand extending longitudinally of said duct.

2. The apparatus of claim 1 wherein said coiled-coil strand abutts the inside wall of said duct at the outer periphery of said coiled-coil strand to provide a thermally conductive joint between the outer periphery of said coiled-coil strand and the wall of said duct.

3. The apparatus of claim 2 wherein said coiled-coil strand is bonded to the inside wall of said duct.

4. The apparatus of claim 1 wherein said coiledcoiled-coil strand is bonded to the inside wall of said duct.

5. The apparatus of claim 2 including, means for passing a cooling fluid through said duct for removing heat from said duct. and from said heat exchanging structure.

6. In a microwave tube: an evacuated envelope; anode and cathode electrode means for producing a stream of electrons within said envelope;

electromagnetic wave energy supportive structure within said envelope in electromagnetic wave energy exchanging relation with said beam of electrons;

said wave supportive structure including, a coolant duct for passage of a fluid coolant therethrough in heat exchanging relation with said duct for transfer of heat from said duct to said fluid, a thermally conductive heat exchanging structure disposed in said duct in heat exchanging relation with the fluid to be passed through said duct and with the walls of said duct for increasing the heat transfer between the walls of said duct and the fluid, said heat exchanging structure comprising a strand of thermally conductive material wound into at least a coiled-coiled-coil configuration, the space between adjacent turns of said coil being substantially unfilled and said coiled-coil strand extending longitudinally of said duct.

7. The apparatus of claim 6 wherein said coiled-coil strand abutts the inside wall of said duct at the outer periphery of said coiled-coil strand to provide a thermally conductive joint between the outer periphery of said coiled-coil strand and the inside wall of said duct. l 

1. In a heat transfer duct: a duct for passage of a fluid therethrough in heat exchanging relation with said duct for transfer of heat therebetween; a thermally conductive heat exchanging structure disposed in said duct in heat exchanging relation with the fluid to be passed through said duct and with the walls of said duct for increasing the heat transfer between the walls of said duct and the fluid; said heat exchanging structure comprising, a strand of thermally conductive material wound into at least a coiledcoiled-coil configurAtion, the space between adjacent turns of said coil being substantially unfilled and said coiled-coil strand extending longitudinally of said duct.
 2. The apparatus of claim 1 wherein said coiled-coil strand abutts the inside wall of said duct at the outer periphery of said coiled-coil strand to provide a thermally conductive joint between the outer periphery of said coiled-coil strand and the wall of said duct.
 3. The apparatus of claim 2 wherein said coiled-coil strand is bonded to the inside wall of said duct.
 4. The apparatus of claim 1 wherein said coiled-coiled-coil strand is bonded to the inside wall of said duct.
 5. The apparatus of claim 2 including, means for passing a cooling fluid through said duct for removing heat from said duct and from said heat exchanging structure.
 6. In a microwave tube: an evacuated envelope; anode and cathode electrode means for producing a stream of electrons within said envelope; electromagnetic wave energy supportive structure within said envelope in electromagnetic wave energy exchanging relation with said beam of electrons; said wave supportive structure including, a coolant duct for passage of a fluid coolant therethrough in heat exchanging relation with said duct for transfer of heat from said duct to said fluid, a thermally conductive heat exchanging structure disposed in said duct in heat exchanging relation with the fluid to be passed through said duct and with the walls of said duct for increasing the heat transfer between the walls of said duct and the fluid, said heat exchanging structure comprising a strand of thermally conductive material wound into at least a coiled-coiled-coil configuration, the space between adjacent turns of said coil being substantially unfilled and said coiled-coil strand extending longitudinally of said duct.
 7. The apparatus of claim 6 wherein said coiled-coil strand abutts the inside wall of said duct at the outer periphery of said coiled-coil strand to provide a thermally conductive joint between the outer periphery of said coiled-coil strand and the inside wall of said duct. 